Optical elements of various types usually require a high degree of perfection for one or more of the optical surfaces. One type of precision surface is a flat or curved lens or mirror surface. Another example is a Fabry-Perot etalon that is used as an interferometer for production of interference fringes in an optical instrument. One such etalon is a high finess etalon formed of a thin polymer film, e.g. 16 microns thick, with a semi-reflective gold coating thereon. Another such etalon is a low finess etalon consisting of a silica plate, e.g. 50 microns thick. A more complex etalon with multiple surfaces is disclosed in U.S. Pat. No. 4,609,822. Etalons, as generally known in the art, provide interference fringes for an instrument via reflections between at least two surfaces of the etalon. The surfaces must be flat and parallel with high precision. It can be difficult to manufacture optical elements with a degree of required perfection. Therefore it is desired to test such elements for variations across the surface or surfaces.
Inspections for contours of optical surfaces are typically effected by way of interference fringes. A monochromatic beam of radiation incident on a sample surface is doubly reflected in such a manner as to produce fringes that are related to contours of the reflecting surface, as disclosed, for example, in U.S. Pat. No. 4,139,302. As illustrated in this reference, the fringe pattern for study of surfaces is commonly recorded by photography for analysis. U.S. Pat. No. 4,169,980 discloses, alternatively, use of video camera detection and circuitry for locating centers of fringes in an interference pattern.
Interferometry for positioning of an object is commonly based on the well known Michelson principles, in which a beam is split into two beams that are reflected and recombined. Differences in beam paths result in fringes. For example, in U.S. Pat. No. 4,436,424, an angled diffraction grating reflects one of the beams and is mounted on a carriage that is moved transversely to vary the reflected path length. A changing fringe pattern thereby provides a measurement of the transverse movement. In the system of U.S. Pat. No. 3,588,462, a pair of diffraction gratings is used to generate a fringe pattern associated with movement. In these applications the diffraction gratings are used for reflecting the radiation and not for their alternative use which is dispersion of radiation into separated wavelengths.
U.S. Pat. No. 3,588,254 teaches a combination of two systems each effecting interferometry of radiation from the same tunable crystal laser. The first system is associated with a positioning table and utilizes a Michelson interferometer with laser input, fringe detection associated with the position of the table, and feedback to a motor to control positioning of the table. The second system incorporates a second interferometer with a motorized carriage reflector, detecting a split-off beam from the laser. Any changes in fringe pattern of the second interferometer are associated with temperature changes in the system. Feedback from the changes in pattern is directed to regulation of the laser frequency with the laser-driving heater, so as to compensate the laser wavelength for the temperature changes.
A solid state diode laser powered with a controllable injection current to vary laser frequency is taught in U.S. Pat. No. 4,410,273. The injection current is varied cyclically, and the laser output is split and directed through sample and reference cells, the two beams being detected separately. A feedback to the current control centers the laser frequency on a associated absorption feature of the reference cell.